FIELD OF THE INVENTION
The present invention relates to implantable medical devices and methods, and more particularly to an implantable pacemaker or pacemaker system that provides a simple and easy-to-implement technique for automatically assessing the capture threshold ("autothreshold") of the implantable pacemaker. Once the capture threshold is known, the stimulus energy may then be automatically adjusted as needed in order to assure that the output stimulus effectuates capture ("autocapture").
It is anticipated that pacemakers will soon employ autocapture and autothreshold features that enable the capture threshold of a given patient to be regularly determined, thereby enabling the output energy of the pacer stimulus to be optimally set to a value that is above the capture threshold, but not too far above the capture threshold. Assuring that the output energy is above threshold assures that capture will occur, while keeping the output energy not too far above threshold conserves the limited power available within the pacemaker battery.
Reliably determining capture within an implantable pacer has heretofore been a formidable task. "Capture" occurs when the applied electrical stimulus generated by the pacemaker is of sufficient energy to stimulate or depolarize the cardiac muscle tissue, thereby causing a cardiac contraction. Capture fails to occur when the applied stimulus is of insufficient energy to stimulate or depolarize the cardiac tissue. Needless to say, for a cardiac pacemaker to properly perform its intended function, it is critically important that the electrical stimuli it issues be of sufficient energy to capture the heart, i.e., to cause the cardiac tissue to depolarize.
The classical approach to determine capture is to apply a ventricular stimulus (V-pulse) to the ventricle of the heart and look for an evoked R-wave response with each and every V-pulse thus applied. The evoked R-wave response is usually monitored between the tip and ring of a bipolar lead connected to the pacemaker sensing circuits. The evoked R-wave response may also be monitored by looking between the ring electrode and pacemaker case. In either event, a bipolar pacing lead has generally been required in order to detect the evoked response.
The evoked R-wave response is monitored immediately (within 5-20 msec) of the pacing pulse (V-pulse) at a time when the polarization voltage, i.e., that voltage which appears at an electrode/tissue interface when an electrical stimulus is applied to tissue through the electrode, is relatively high. In order to avoid detecting the polarization voltage and classifying it as an evoked R-wave response, it is necessary to use low polarization materials in the electrode. Further, a calibration algorithm is typically needed in order to set an evoked R-wave response threshold voltage since the evoked R-wave response and polarization voltage occur simultaneously. Thus, when the polarization voltage is very high, it may not be possible to reliably detect an evoked R-wave response.
Assuming that an evoked R-wave response can be detected, a high output backup pulse may be applied when no evoked R-wave response from a primary pacer pulse is detected (i.e., when loss of capture occurs). Should two (2) consecutive loss-of-capture events occur, an autocapture routine of the pacer then incrementally increases the amplitude of the primary stimulus until capture occurs as determined by detecting an evoked R-wave response from the new increased-energy primary stimulus.
An autothreshold algorithm may also be periodically invoked, e.g., once or twice a day, during which the pacer decrements the amplitude of its output stimulus until capture is lost (no evoked response). The output is then incrementally increased until capture is regained. Every loss of capture primary stimulus is followed by a high output back-up stimulus in order to maintain the cardiac rhythm of the patient.
All prior art autocapture and autothreshold schemes require a bipolar pacing system, or at least a bipolar sensing configuration. It has thus not heretofore been possible to reliably provide the autocapture and autothreshold features when only monopolar pacing is employed. Further, autocapture and/or autothreshold may not work with all pacing leads (i.e., leads that exhibit high polarization potentials). Additionally, verifying capture with every pacing pulse may require significant overhead in the pacing circuitry in order to assure that a polarization voltage is not incorrectly detected as an evoked R-wave response.
What is needed, therefore, is an improved way to reliably assess the capture threshold. In particular, what is needed is a capture-assessment technique that does not require the evoked R-wave response to be sensed, and that thus eliminates the problems associated with distinguishing the evoked R-wave response from polarization voltages, and avoids the need of using low polarization materials in the electrode.